This invention relates to optical cables for use in optical transmission systems, and especially to cables for use in overhead optical transmission systems in which the cable is supported along the route of the system by means of towers, masts or other upstanding supports that are also employed to support electrical power cables.
In systems of this kind it is the general practice to earth the optical cable or cables at the towers, masts or other supports (hereinafter simply referred to as towers). When the electrical power lines are on load, electrical currents may be capacitively induced on the optical cable due to the distributed capacitance between the cable and the power lines. The voltage induced on the optical cable will reach a maximum at mid span between the towers, while the current flowing along the cable will be greatest in the region of the towers. Under dry conditions the induced currents will be relatively small due to the relatively high longitudinal resistance of the cable, e.g. in the region of 10.sup.12 Ohm m.sup.-1 but under wet conditions when the surface resistance of the cable is much lower, e.g. in the region of 10M Ohm m.sup.-1, much higher currents will be induced. Joule heating of the cable surface by the induced currents can cause a short length of the cable surface to become dry, usually in the region of a tower where the current is highest. When this happens the major part of the induced voltage on the cable is dropped across the short dry band due to its high longitudinal resistance, and so called "dry-band arcing" may occur which can cause severe damage to the cable.
It is possible to overcome the problem of dry-band arcing by providing the optical cable with a longitudinally extruding electrically conductive element. However, it is possible in some circumstances for the optical cable to touch one of the power conductors or to become sufficiently close to the power conductor to allow flash over to occur, referred to herein as "clashing". This may occur after the cable has been installed, especially under windy conditions or if the cable is carrying considerable ice which will make it stretch, with the result that the conductive element will be burned away or evaporate. Clashing of the optical cable with a power line may also occur during installation of an optical fibre cable on to an electrical transmission system that is on load, with the consequent risk to the safety of the installation personnel if the optical cable is conductive. One solution to the latter problem is to install a conventional All Dielectric Self Supporting (ADSS) optical cable and then wrap a conductor around the installed cable in situ. Such a procedure, however, has the disadvantage that it requires two separate installation operations with consequent increase in expense, and does not overcome the problem of clashing of the installed cable. Another solution to the problem of dry-band arcing is described in International Patent Application No: WO 91/16648 of NKF in which dry-band arcing is prevented by providing the cable with a series of capacitively coupled elements along its length. However, in order for the elements to have the appropriate degree of coupling, they are formed as overlapping annular bands along the cable, which are relatively difficult to form by conventional cable forming processes.